The impetus for this Program Project on Genetic Approaches to Cardiac Development arises from the indispensable importance of cardiac organogenesis for survival for survival even during embryonic life, and from the conviction that fundamental knowledge gained from studies of cardiac development in model organisms is ultimately essential to understanding the failure of normal cardiac morphogenesis, manifested as congenital heart disease. Our overall theme remains a genetic analysis of cardiac development, tightly focused on the use of mouse genetics. In addition to more conventional approaches (gain-of- function mutations in transgenic mice and loss-of-function mutations through homologous recombination), we have already made heavy use of promoter mapping in transgenic mice, investigated cardiac-lethal transgenes in F/O embryos, used yeast artificial chromosomes to rescue mutant mice, implemented cardiac-restricted knockout mutations by directing Cre recombinase to the developing heart, and used a further refinement of the Cre/lox system for chromosome engineering. To a greater extent than before, these approaches allows us not only to investigate the mechanisms for cardiac myocyte differentiation, but also for cardiac myogenesis itself and later morphogenesis-aspects that cannot be adequately modeled by cell culture methods alone. The major topics under study are: the role of NK-2 related genes in directing early embryonic cardiac cell differentiation; the role of the GATA family in cardiogenesis; identification of the inv gene in left-right looping; the role of TGFbeta family in growth factor signaling during cardiac development and deciphering the DiGeorge locus 22q11 involved with the neural crest dependent cardiovascular development.